1. Field of the Invention
The present invention relates to a semiconductor module, in particular, to a semiconductor module provided with semiconductor devices on both surfaces of its substrate.
2. Description of the Related Art
Recently, with the trend toward miniaturization and high performance of electronic devices, there is a demand for reduction of the size of circuit devices used in electronic devices. As a way to achieve this, it can be considered that an area of a circuit device is reduced by arranging devices on both surfaces of its substrate.
Wiring of a substrate is generally formed of Cu, the coefficient of thermal expansion of which is greatly different from that of a resin filled between the wirings. If the wirings are formed so as to have a uniform pattern across the whole substrate, a stress occurring due to a difference between the coefficients of thermal expansion of the wiring and the resin during a heat treatment, is generated uniformly across the whole substrate. Therefore, it is thought that the substrate does not deform greatly.
However, wiring patterns in respective wiring layers including the surface and the underside of a substrate are generally different from each other, and in many cases, each wiring pattern is not formed so as to be symmetric in a plane. Accordingly, when some sort of heat is applied to such a substrate, a stress occurring at each area of the substrate is not equal to each other, causing the substrate to deform and warp in a complicated way.
In contrast, a semiconductor device mounted on a substrate generally has a higher stiffness than that of the substrate, and hence warp of the substrate can be suppressed. In particular, the semiconductor devices provided on both surfaces of a substrate can further enhance the stiffness of the substrate. However, even in this case, the warp of the substrate is likely to occur in a direction depending on the shape or arrangement of the semiconductor devices.